Study on Fracture Strength Assessment (2nd. Report)

Abstract

Japan Welding Engineering Society (WES) standard WES 2805 and British Standard Institution (BSI) standard BSI PD6493 were established in 1980. Since then, these standards have been widely used to evaluate fracture safety of structures. They are based on elastic-plastic fracture mechanics (CTOD criterion), because most brittle fracture may occur from defect at structural stress concentration where plastic zone size generally seems to be larger than the defect size.Work on revision to these standards are underway in U. K. and Japan. The BSI are going to provide a new defect assessment. The fracture sections of the revised document will take three levels of assessment. Level 1 is essentially consistent with the PD6493-1980 and treated as a preliminary evaluation. In Level 2 and 3 assessments, brittle fracture and plastic collapse are taken into account by use of a failure assessment diagram (FAD) similar to that proposed originally in CEGB R-6 procedure.In Japan, research on the validity of WES 2805 has been carried out and a modified CTOD design curve and a new method for estimating local strain at stress concentrator were proposed. Work on revisions to WES 2805 has been in progress by taking into account the recent advances in fracture mechanics treatment.In this paper, critical evaluation was carried out for fracture assessment of CTOD and FAD. Wide plate tests with cracks emanating from a hole were carried out and the fracture strength was evaluated by means of WES CTOD and FAD approaches. The FAD analysis based on the WES CTOD was also investigated. The assessment results were almost the same between those two approaches. However, it is said that the WES CTOD method is more practical, because this method is based on the relation between CTOD and local strain. In some practical cases, only the local strain near the region where defect is discovered, is available. The FAD approach is required the global stress condition (plastic collapse load) for fracture evaluation.